(CNN) -- Tokyo's governor dispatched a team Sunday to survey a set of islands in the East China Sea to which both Japan and China lay claim.
China's state-run media immediately declared the survey "illegal."
The incident is the latest in rising territorial tensions in North Asia.
The uninhabited islands are known in Japan as Senkaku and in China as Diaoyu, and are privately-owned by a Japanese family.
But both China and Japan separately claim them as part of their territory. The islands, located between Taiwan and Okinawa, sit among rich fishing waters and are also believed to be rich in oil resources.
For now, the Japanese government leases the island from the family to "peacefully and stably maintain" them, as Chief Cabinet Secretary Osamu Fujimura said.
But Tokyo Governor Shintaro Ishihara has begun talks with the private owners and said he expects to strike a deal to buy the islands by the end of the year.
"In place of a weak-kneed national government that has failed to act from start to finish, Tokyo will consider how to make the most of the fertile seas and abundant nature of these islands," Ishihara said earlier this year.
The team dispatched Sunday sailed on a ship and then rode on rubber boats to survey the shoreline. It did not set foot on the islands.
Animosity has run deep between China and Japan over the islands.
They are symbolic of what many in China see as unfinished business, redressing the impact of Japanese occupation in the 1930s and 1940s.
China says its claim on Diaoyu extends back hundreds of years. Japan says China ceded sovereignty when it lost the Sino-Japanese war in 1895.
Japan then sold the islands in 1932 to descendants of the original settlers.
Japan's surrender in World War II clouded the issue again.
The islands were administered by the U.S. occupation force after the war. But in 1972, Washington returned them to Japan as part of its withdrawal from Okinawa.
That, says Chinese analyst Guo Xiangang, is where the current troubles begin.
"The U.S. handed over the island to Japan for its own purpose during the Cold War. So, personally, I think the U.S. should take the blame for the dispute of Diaoyu island," said Guo, the deputy director of the China Institute of International Studies, a think tank connected to China's Ministry of Foreign Affairs.
"There shouldn't be any discussion on Diaoyu Island. It has always been a part of China," he said.
In August, a diplomatic row erupted between Japan and China after a group of Chinese nationals were photographed raising flags there. They were arrested.
Japanese activists too have swum ashore, raising their flag, in incidents in 1990 and 1996.
In 2010, tensions rose to a boiling point when a Chinese fishing trawler rammed into a Japan Coast Guard vessel on patrol in the islands' waters.
Japan detained the crew members but later released them under Chinese diplomatic and trade pressure.